Meaning
Dimensionless fluid physics characterizes the ratio between viscous drag forces and interfacial tension forces acting upon a boundary. This capillary number quantifies how flow velocity and material viscosity overcome the surface tension that keeps fluids from wetting or displacing one another. High values represent systems dominated by viscous forces where liquid shapes deform readily, while low values describe regimes where surface tension enforces a rigid interface.
Moulding Dynamics
Injection moulding processes rely upon this metric to predict how polymer melts fill complex geometric cavities. The capillary number dictates the stability of the moving front as resin traverses narrow flow channels or textured surfaces. When a processor increases injection speed, the resulting shear forces modify the balance between surface energy and friction.
Sudden drops in this ratio during high speed filling sequences cause gas entrapment or incomplete surface reproduction known as short shots.
Resin Mechanics
Molecular properties of the polymer influence the range of available motion during the cooling phase inside the tooling. Operators observe that additives intended to improve flow characteristics modify the viscosity component of the capillary number to assist in thin wall filling. Differences between virgin pellets and regrind material change the rheological profile of the melt, creating inconsistent results across different production batches.
Engineers define acceptable processing windows by monitoring these shifts to ensure the internal pressure profile remains stable during the packing stage of the cycle.
Interface Control
Precise management of the interaction between molten plastic and tool steel determines the aesthetic and functional quality of the finished part. Surface roughness on the tool creates a mechanical resistance that interacts with the capillary number to trap air if the fluid front lacks sufficient momentum to displace it. Low velocity filling regimes frequently fail to drive the melt into micro features because the interfacial tension holds the front in a spherical shape rather than a conformal one.
Optimal process settings maintain a consistent force balance to prevent premature solidification before the mould cavity reaches full volume.
